1. Field of the Invention
The present invention relates generally to a double-balanced mixer, and more particularly to a double-balanced mixer which is suitable for mass production and has a stable high frequency characteristic.
2. Description of the Prior Art
A conventional double-balanced mixer used with an automobile telephone and so on, as illustrated in FIG. 9 , is constructed with ferrite cores FC.sub.1, FC.sub.2 having a width of, e.g., 5.2 mm, a depth of, e.g., 3 mm, and a height of ,e.g., 2 mm, which ferrite cores have through-holes CH.sub.1, CH.sub.2 1.2 mm in diameter in which lead wires W.sub.1 -W.sub.3, W.sub.4 -W.sub.6 0.15 mm in diameter are manually wound to form input/output coils LA, LB, which coils are combined with a diode matrix DM to make a circuit. In operation of the double-balanced mixer as set forth in the above, the windings of the coils LA and LB comprise primary coils L.sub.1, L.sub.6 and secondary coils L.sub.2, L.sub.3, L.sub.4, L.sub.5, respectively, and silver electrodes P.sub.5, P.sub.6, P.sub.7, P.sub.8 of the diode matrix DM are connected to one ends of the respective secondary coils L.sub.2, L.sub.3, L.sub.4 , L.sub.5, which have second ends connected to the silver electrodes P.sub.2, P.sub.4, P.sub.9, P.sub.10 as intermediate leads.
These coils comprise a RF side into which a RF signal f.sub.1 is input from the silver electrodes P.sub.1 and P.sub.2, a local side into which a local signal f.sub.2 is input from the silver electrodes P.sub.11, P.sub.12, and an IF side from which an IF signal f.sub.3 is output from the silver electrodes P.sub.3, P.sub.4 and P.sub.9, P.sub.10. Then it is necessary to handle carefully the coils to prevent tearing of the insulation on the lead wires and shorting out of the three windings in the through-holes CH.sub.1 and CH.sub.2.
Further, it is hard to mechanize the process for winding the lead wires W.sub.1 -W.sub.3 and W.sub.4 -W.sub.6 and the tests on completion such as visual examination and characteristic testing require many steps.